1. Field of the Invention
The present invention relates to an ink jet printing head having a structure that the portion of a pressure generating chamber communicating with a nozzle opening for ejecting ink droplets is formed by an elastic plate and a piezoelectric element is formed on the surface of the elastic plate, so that ink droplets eject by the displacement of the piezoelectric element and elastic plate.
2. Related Art
There has been known a conventional ink jet printing head having pressure generating chambers communicating with nozzle openings for ejecting ink droplets and structured such that a portion of the pressure generating chambers is formed by an elastic plate. The elastic plate is deformed by a piezoelectric element to apply pressure to ink in the pressure generating chambers so as to eject ink droplets through the nozzle openings. There are two different types of ink jet printing heads practical used, one structure comprises a piezoelectric element having a vertical oscillation mode in which the piezoelectric actuator is expanded/contracted in the axial direction thereof; and the other structure comprises a piezoelectric actuator having a deflection oscillation mode.
The former structure is able to vary the capacity of the pressure generating chamber when the end surface of the piezoelectric actuator is brought into contact with the elastic plate. Thus, a head adapted to a high density printing operation can be manufactured. However, a difficult process is required in which the piezoelectric actuator is sectioned into comb shape to coincide with the arrangement pitch of the nozzle openings. Moreover, a process is required to locate the sectioned piezoelectric actuator to be located and secured in the pressure generating chamber. Thus, the former structure has a problem in that the manufacturing process is too complicated.
On the other hand, the latter structure is able to provide the piezoelectric element for the elastic plate by a relatively simple process in which a green sheet of the piezoelectric material is applied to be adaptable to the shape of the pressure generating chamber and then the green sheet is baked. Since the deflection oscillation is used, a somewhat large area is required. Thus, there arises a problem in that dense arrangement cannot easily be realized.
To overcome the problem experienced with the latter printing head, a structure has been disclosed in Unexamined Japanese Patent Application (OPI) No. 5-286131. In this case, a uniform piezoelectric material layer is formed on the overall surface of the elastic plate by a film forming technology. Then, the piezoelectric material layer is cut into a shape adaptable to the pressure generating chamber by a lithographic method. Thus, the piezoelectric element is formed independently for each of the pressure generating chambers.
As a result, the process for applying the piezoelectric element to the elastic plate can be omitted. Thus, the piezoelectric actuator may be provided by the lithographic method which is a precise and simple method. Moreover, advantage is realized in that the thickness of the piezoelectric actuator is reduced and thus high speed operation is permitted. In this case, at least only the upper electrode is provided for each of the pressure generating chambers while the piezoelectric material layer is as it is provided for the overall surface of the elastic plate. Thus, the piezoelectric actuator corresponding to the pressure generating chambers can be operated.
The printing head of the type comprising the piezoelectric actuators having the deflection mode is arranged such that the piezoelectric actuators corresponding to the pressure generating chambers are covered with an insulating layer. Moreover, windows (hereinafter called "contact holes") are provided for the insulating layer so as to form connection portions with a conductive pattern for supplying voltage to each of the piezoelectric actuator such that the windows are formed to correspond to the pressure generating chambers. Moreover, connecting portions between the piezoelectric actuators and the conductive pattern are formed in the contact hole.
However, the contact hole portion in which the connection portion between the piezoelectric actuators corresponding to the pressure generating chambers and the conductive pattern is formed easily encounters generation of great stress because of the operation of the piezoelectric actuators. Thus, there arises a problem in that generation of cracks and occurrence of breakage cannot be prevented.
Since the connection portion with the conductive pattern is connected to the contact hole portion, displacement caused because of application of voltage is relatively restrained. However, the compliance is not small as compared with that of the other portions. Therefore, there arises a problem in that the eject speed is reduced and the operating voltage is raised.
The foregoing problem becomes critical when the piezoelectric material layer is formed by the film forming technology. Since the piezoelectric material formed by the film forming technology is very thin, only a poor rigidity can be realized as compared with the structure formed by applying the piezoelectric actuator.